Related derivatives of 1,2,3,6-tetrahydropyridine have been described by Knaus et al, in J. Het. Chem. 13, 1237 (1976) and in U.S. Pat. No. 4,088,653 issued May 9, 1978 (which corresponds to Knaus et al., Canadian Patent Application No. 283,573 filed July 27, 1977).